Coated aspirin may lose some of its cardiovascular benefits because it takes longer for the drug to dissolve into the bloodstream, a new study suggested.

The study of 400 healthy volunteers was designed to uncover whether certain people were resistant to aspirin's blood-thinning effects. Millions of men and women take aspirin daily to prevent blood clots that cause heart attacks and strokes.

Instead, the authors from the University of Pennsylvania pointed to the stomach-soothing coating sometimes applied to aspirin as a culprit.

Brand-name aspirin maker Bayer, which partially funded the study, took issue with some of the conclusions.

"The authors' suggestion that use of enteric coated aspirin should be questioned, based on these study results, is of concern given the study population and methodology used, neither of which reflect real-world clinical use," spokeswoman Anne Coiley said in a statement to ABC News.

For people with a high risk of heart attack or stroke, studies suggest an aspirin a day can cut the risk.

If a person is experiencing a heart attack, it is believed chewing an aspirin can buy time by thinning the blood while help is on the way.

"Chew the aspirin, don't just swallow it," said ABC News senior health and medical editor Dr. Richard Besser. "You need that anti-clotting effect in your bloodstream as quickly as possible. If you have uncoated aspirin, that's best. If not, take a coated aspirin. But remember: Chew it, don't swallow it."

Whether you take aspirin to ease a headache or to help fend off heart disease, uncoated pills dissolve faster in your stomach.

While coated aspirin takes longer to dissipate, mostly in your lower digestive tract after it leaves your stomach, it does mean fewer stomach ulcers and less stomach upset -- good for people with sensitive stomachs.

"But for primary prevention of heart attack and strokes, I agree with the study findings, and it is very interesting to see this and be aware of this," said Dr. Martha Gulati, associate professor of medicine in at Ohio State University's Wexner Medical Center in Columbus.

Daily aspirin -- coated or not -- can help prevent heart disease, Besser said. He encouraged people at risk to carry the drug at all times.

"You can even get one of these key-ring gizmos so that you have an aspirin with you all the time," he said. "My dad carries one of these."